use fewer layers and remove apt cache

also use full-upgrade to ensure all conflicts are resolved

Dockerfile

@@ -11,7 +11,7 @@ LABEL "about"="AFLplusplus docker image"
 
 ARG DEBIAN_FRONTEND=noninteractive
 
-RUN apt-get update && apt-get upgrade -y && \
+RUN apt-get update && \
     apt-get -y install --no-install-suggests --no-install-recommends \
     automake \
     bison flex \
@@ -23,24 +23,24 @@ RUN apt-get update && apt-get upgrade -y && \
     wget vim jupp nano bash-completion \
     apt-utils apt-transport-https ca-certificates gnupg dialog \
     libpixman-1-dev \
-    gnuplot-nox
+    gnuplot-nox \
+    && rm -rf /var/lib/apt/lists/*
 
 RUN echo deb http://apt.llvm.org/focal/ llvm-toolchain-focal-11 main >> /etc/apt/sources.list && \
     wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - 
 
 RUN echo deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu focal main >> /etc/apt/sources.list && \
     apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 1E9377A2BA9EF27F
-
-RUN apt-get update && apt-get upgrade -y
 
-RUN apt-get install -y gcc-10 g++-10 gcc-10-plugin-dev gcc-10-multilib \
-    libc++-10-dev gdb lcov
-
-RUN apt-get install -y clang-11 clang-tools-11 libc++1-11 libc++-11-dev \
+RUN apt-get update && apt-get full-upgrade -y && \
+    apt-get -y install --no-install-suggests --no-install-recommends \
+    gcc-10 g++-10 gcc-10-plugin-dev gcc-10-multilib libc++-10-dev gdb lcov \
+    clang-11 clang-tools-11 libc++1-11 libc++-11-dev \
     libc++abi1-11 libc++abi-11-dev libclang1-11 libclang-11-dev \
     libclang-common-11-dev libclang-cpp11 libclang-cpp11-dev liblld-11 \
     liblld-11-dev liblldb-11 liblldb-11-dev libllvm11 libomp-11-dev \
-    libomp5-11 lld-11 lldb-11 llvm-11 llvm-11-dev llvm-11-runtime llvm-11-tools
+    libomp5-11 lld-11 lldb-11 llvm-11 llvm-11-dev llvm-11-runtime llvm-11-tools \
+    && rm -rf /var/lib/apt/lists/*
 
 RUN update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 0
 RUN update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-10 0